Dynodes for electron discharge tubes and methods of making same



April 14, 1964 w. wlLcocK 3,128,531

DYNoDEs FOR ELEcTRoN DISCHARGE TUBES AND METHODS OF' MAKING SAME Filed Oct. 5, 1960 2 Sheets-Sheet 1 INVENTOR //LL/AM ESL/E l//LCOCK Aprll 14, 1964 w. wlLcocK 3,123,531

l DYNODES FOR ELECTRON DISCHARGE TUBES AND METHODS OF MAKING SAME Flled Oct 5, 1960 2 Sheets-Sheet 2 INVENTOR MAL/AM ESL/E MLCOCK United States Patent O 3,128,531 DYNODES FOR ELECTRON DISCHARGE TUBES AND METHODS GF MAKING SAME William Leslie Wilcock, East Molesey, England, assignor to National Research Development Corporation, London, England, a British corporation Filed Oct. 5, 1960, Ser. No. 60,643 Claims priority, application Great Britain Oct. 22, 1959 13 Claims. (Cl. 29-25.17)

This invention relates to improvements in or relating to certain types of electron discharge tubes, and is especially applicable to image intensifier tubes.

In one type of image intensifier tube, there is positioned between a photocathode and an anode at least one, and more usually a number, of dynodes which promote electron multiplication without loss of the relative spatial distribution of the electrons representing the image to be intensified. Each dynode is in the form of a thin film which consists of one or more layers one of which is of a substance which is a good emitter of secondary electrons and is positioned in the tube in a plane parallel to the plane of the photocathode. An electron lens is provided so arranged as to focus the electron image radiated from the photocathode or from a previous dynode on to the dynode so that a new electron image will be generated in the form of a transmitted secondary electron emission from the dynode. The essential requirements of such a dynode are that it should have good secondary emission properties and that it should have adequate conductivity so that it may be maintained at a potential appropriate to its position in the tube and it is also desirable that it should have sufiicient mechanical strength to be selfsupporting. It Imust also be capable of withstanding the baking temperatures of the order of 300-400 C. to which it is normally subjected during evacuation of the tube.

No known secondary emission material possesses all these properties in sufficient measure to form a satisfactory film dynode by itself, and it has therefore been suggested that a composite film should be used. Such films have been tried using a comopsite film of aluminium oxide, to provide the material strength, aluminium to provide the conductivity and potassium chloride as a secondary emission material. The lm is stretched across a frame of suitable material. A difficulty arises with these films, however, that, on baking of the tube and subsequent cooling, they tend to tear away from the frame.

A careful investigation of this phenomenon has lead to the conclusion that this breaking is due to the fact that a potassium chloride filrn, on being baked at 300 C. and then cooled, contracts from its original size and that this contraction sets up stresses in the aluminium oxide which results in its failure. It is believed that this contraction is due to recrystallisation of the potassium chloride. It seemed, therefore, that to prepare successfully a film dynode capable of resisting temperatures of up to 300 C., it would be necessary to make some allowance for this shrinkage of the secondary emitter, and the present invention is concerned with expedients to this end.

According to the invention, therefore, there is provided a method of making dynodes for incorporation in electron discharge tubes, said dynodes consisting of an outer frame and a composite film supported by said outer frame, said composite film including an electron-permeable supporting layer and a layer of secondary emission material said method comprising the steps of producing on the supporting frame a slack, thin, supporting film, forming on one surface thereof a coating of secondary emission material, said coating of secondary emission material being of smaller dimensions than the area enice closed within the frame and being so situated Within the frame that a margin is left between the outer edge of the secondary emission material and the inner edge of the frame so that, on subsequent baking, at least some of the slack in the supporting film is taken up by shrinkage of said electron emission material. It is, of course, necessary that the film should be conducting and therefore it will be understood that, where either the electron-permeable supporting layer or the secondary emission material is not itself sufficiently conductive, an extra layer of conductive material must be added to render the film conducting. Where a layer of conductive material is provided, it must be in direct contact with the layer of secondary emission material.

In a preferred embodiment of the invention, the step of producing said slack, thin supporting film on said supporting frame comprises mounting said film taut on said frame and then stretching said lm so that it acquires a permanent slackness. According to one aspect of the invention, this is carried out by choosing the materials of the film and the frame so that the thermal co-efiicient of expansion of the material of said thin, supporting film is less than the thermal co-eflicient of expansion of said supporting frame and then stretching said film so that it acquires a permanent slackness by heating said film and supporting frame to a temperature such that the expansion of said frame relative to said thin, supporting film stretches said film past the yield point thereof and then cooling said film and said frame. By yield point of the lm is to be understood that point in the stretching of the film beyond which the film must be stretched in order that it will not contract to its original dimensions on release of the stretching tension. The frame across which the film is supported may be of any material suitable for use in high vacuum equipment and capable of withstanding the temperatures encountered in the baking of the tube. When the above heating and cooling method is used for producing said slack, thin, supporting film, however, it has been found convenient to use a soda glass ring as the supporting frame along with a material chosen from the group consisting of aluminium oxide and magnesium oxide as the thin supporting film. Aluminum oxide has been found to be especially suited. Contrary to what might be expected from a knowledge of its bulk properties, it transpires that aluminum oxide in the form of a thin film has a yield point as hereinbefore defined and, as aluminium oxide has a 'smaller thermal co-efiicient of expansion than has soda glass, a slack film of aluminium oxide can be provided by suitable heat treatment. Alternatively, a metal such as aluminium could be used as the thin, supporting layer in which case it would be necessary to select a material, preferably a metal, for the supporting frame which has a larger thermal co-eiiicient of expansion than the metalused as the supporting layer.

As regards secondary emission materials, the best known and most suitable are the alkali metal halides and the alkaline earth metal halides, especially potassium chloride.

In a preferred embodiment of the invention, the method of making dynodes comprises the steps of producing on a soda glass ring, a slack, thin supporting film of aluminium oxide, forming on the surface of said film a coating of aluminium'and thereon a coating of potassium chloride, said coating of potassium chloride being concentrically disposed and of smaller dimensions than the area enclosed within the soda glass ring to provide a margin between the outer edge of said potassium chloride and the inner edge of said lsoda glass ring whereby, on subsequent baking, at least some of the slack in said supporting film is taken up by shrinkage of said potassium chloride.

In this method the step of producing said slack, thin film of aluminium oxide on said soda glass ring comprises mounting said thin, supporting film of aluminium oxide taut on said soda glass ring and then stretching said aluminium oxide film so that it acquires a permanent slackness. In this method the step of stretching said aluminium oxide film so that it acquires a permanent slackness comprises heating said aluminium oxide lm and said Soda glass ring to a temperature such that said aluminium oxide film is stretched past the yield point thereof. The temperature to which the aluminium oxide film and soda glass ring are heated in order to effect the stretching is preferably in the range of about 300 C. to 500 C.

By preparing dynodes according to the method as hereinbefore described, any contraction in the secondary emission layer of the film due to heating of the dynode is accommodated by the slack in the thin, supporting layer and thus no undue stresses are set up in other layers of the film.

Further according to the invention there is provided a dynode suitable for incorporation in electron discharge tubes, said dynode comprising a supporting frame, a slack thin supporting film securely attached across said frame, said supporting film being of an electron permeable material, and a coating of secondary emission material on one surface of said film, said secondary emission material being marginally spaced from said supporting frame, whereby when the dynode is subjected to high temperatures during its incorporation in an electron tube at least some of the slack in said supporting film is taken up by shrinkage of the coating of said secondary emission material. The supporting frame is conveniently circular in shape. Preferably the thermal co-efficient of expansion of said film is less than that of the metal of said supporting frame.

In one embodiment of the invention said dynode comprises a supporting frame, a slack, thin supporting film attached across said supporting frame, said fiim consisting of material chosen from the group of aluminium oxide and magnesium oxide, and a coating of secondary emission material on said film, said secondary emission material being selected from the group consisting of alkali metal halides and alkaline earth metal halides, said secondary emission material in area being of smaller dimensions than the area enclosed by said frame to leave a margin between it and the frame.

In a further embodiment of the invention, said dynode comprises a soda glass circular ring supporting frame, a slack thin supporting film attached across said frame, said film being comprised of a layer of aluminium oxide, a layer of conductive material and on said layer of conductive material a coating of potassium chloride said potassium chloride coating being concentrically disposed within said frame and of smaller dimensions to provide an annular margin between its outer edge and the inner edge of said frame. Preferably said conductive material is aluminium.

In a particular embodiment of the invention, therefore, said dynode comprises a circular ring supporting frame, a slack thin supporting lm securely attached across said frame, said lm being comprised of aluminium oxide having a coating of aluminium thereon, and a coating of a secondary emission material on said film, said secondary emission material being disposed concentrically within said ring frame and of smaller diameter to leave a margin between it and the frame, whereby on subsequent baking during incorporation in an electron tube at least some of the slack in said supporting film is taken up by shrinkage of the coating of said secondary emission material.

Further according to the invention, there is provided a method of making an electron discharge tube having incorporated therein at least one film type dynode said method comprising the steps of attaching across a supporting frame a slack, thin supporting film, forming on one surface thereof a coating of secondary emission material, said coating of secondary emission material being of smaller dimensions than the area enclosed within the frame and being so situated within the frame that a margin is left between the outer edge of the secondary emission material and the inner edge of the frame, mounting said structure Within an electron discharge tube, and baking the tube to outgas it and at the same time to take up at least some of the slack in said supporting iilm by shrinkage of the coating of secondary emission material, and sealing said tube.

In one embodiment of the invention, said method comprises the steps of attaching across a soda glass circular ring supporting frame a thin film of aluminium oxide, coating said aluminium oxide with aluminium, providing .slack in said composite film by heating the same beyond 1ts yield point and then permitting the same to cool, thereafter forming on one surface of said composite film a coating of potassium chloride, and spacing said coating from said frame to provide annular margin between its outer edge and the inner edge of said frame, mounting said structure within an electron tube, and baking said tube while evacuating the same, whereby the tube is outgassed and the potassium chloride coating layer is shrunk so that at least some of the slack in said composite film is taken up, and sealing said tube.

The invention will now be further described in relation to a particular embodiment thereof, illustrated in the accompanying drawings, in which:

FIGURES 1 4 show cross-sectional views of a dynode in the various stages of preparation as hereinafter described.

FIGURE 5 shows a plan view of the completed dynode.

FIGURE 6 shows a plan View of an electron discharge tube having incorporated therein a number of film type dynodes at a stage during the manufacture thereof.

Across a soda glass ring l, approximately 2 x l mm. in cross section and of 2.5 cms. diameter, was stretched a thin aluminium oxide film 2. The film was approximately 450 A. thick and was prepared by anodising on aluminium sheet by a method known in the art. It was stuck to the ring with potassium silicate. The stage illustrated in FIGURE l had then been reached.

The ring and film were then baked in an oven at 460 for a few minutes and then allowed to cool. The resultant film was slack and appeared puckered in form as illustrated in FIGURE 2.

A thin film of aluminium 3 was then evaporated on to the surface of the aluminium oxide, and on to the composite film so formed was deposited, again by means of vacuum evaporation, a coating of potassium chloride 4. A marginal region of the aluminium oxide film was masked during this second evaporation so as to restrict the area covered by the potassium chloride coating and leave a margin between the inner circumference of the soda glass ring and the outer circumference of the potassium chloride layer. Both the aluminium coating and the potassium chloride coating followed the contours of the aluminium oxide as illustrated in FIGURE 3.

A number of dynodes prepared as above were then inserted into an electron discharge tube (illustrated in FIGURE 6). The discharge tube consisted of a cylindrical tube 5 at opposite ends of which were situated a photocathode plate 6 and a liuorescent screen 7. The dynodes f5 were inserted at intervals along the tube, each dynode being xed in a position transverse to the axis of the tube by a mounting wire 9 sealed into the wall of the tube. In addition to the dynodes the tube was provided in the usual way with a number of electrodes 10 which, during the operation of the tube, are maintained at different potentials relative to each other so that a suitable potential gradient exists along the length of the tube. When the tube is arranged for operation, there is also provided a conventional electron lens such as the coil 11.

After insertion of the dynodes 8, the tube was evacuated. During the evacuation, the tube was baked at 300 C. for 10 hours and, on cooling, it was found that the potassium chloride coating on the dynodes had shrunk, and the composite lm structure had taken on the form illustrated in FIGURE 4. The central area carrying the potassium chloride layer was flat while the annular portion of the aluminium and aluminium oxide films between the ring and coating of potassium chloride could then be seen to be tightly puckered in close radial creases as shown in FIGURE 5 indicating a state of tension due to the contraction of the central area. After baking, the tube 5 was sealed off at the outlet pipe 12.

The values hereinbefore given for the dimensions of the dynode, and the temperature and length of time of baking are to be understood as being purely by way of example, and may be varied according to the composition and requirements of the dynode which is to be measured.

I claim:

1. A method of making dynodes for incorporation in electron discharge tubes, said dynodes consisting of an outer frame and a composite film supported by said outer frame, said composite film including an electron-permeable supporting layer and a layer of secondary emission material, said method comprising the steps of producing on the supporting frame a slack, thin, supporting film, forming on one surface thereof a coating of secondary emission material, said coating of secondary emission material being of smaller dimensions than the area enclosed within the frame and being so situated within the frame that a margin is left between the outer edge of the secondary emission material and the inner edge of the frame so that, on subsequent baking, at least someA of the slack in the supporting film is taken up by shrinkage of said electron emission material.

2. A method of making dynodes according to claim 1 wherein the step of producing said slack, thin supporting film on said supporting frame comprises mounting said film taut on said frame and then stretching said film so that it acquires a permanent slackness.

3. A method of making dynodes according to claim 2 wherein the thermal co-eiicient of expansion of the material of said thin, supporting film is less than the thermal co-eticient of expansion of said supporting frame and the steps of stretching said film so that it acquires a permanent slackness comprises heating said film and supporting frame to a temperature such that the expansion of said frame relative to said thin, supporting film stretches said film past the yield point thereof and then cooling said film and said frame.

4. A method of making dynodes according to claim 1 wherein said supporting frame is a soda glass ring and said thin, supporting lm consists of a material chosen from the group consisting of aluminium oxide and magnesium oxide.

5. A method of making dynodes according to claim 1 wherein said thin, supporting layer consists of aluminium oxide.

6. A method of making dynodes according to claim l wherein said coating of secondary emission material consists of a material selected from the group consisting of alkali metal halides and alkaline earth metal halides.

7. A method of making dynodes according to claim 1 wherein the secondary emission material is potassium chloride..

8. A method of making dynodes for incorporation in electron discharge tubes comprising the steps of producing on a soda glass ring, a slack, thin supporting film of aluminium oxide, forming on the surface of said film a coating of aluminium and thereon a coating of potassium chloride, said coating of potassium chloride being concentrically disposed and of smaller dimensions than the area enclosed within the soda glass ring to provide a margin between the outer edge of said potassium chloride and the inner edge of said soda glass ring whereby on subsequent baking, at least some of the slack in said supporting film is taken up by shrinkage of said potassium chloride.

9. A method of making dynodes according to claim 8 wherein the step of producing said slack, thin film of aluminium oxide on said soda glass ring comprises mounting said, thin, supporting film of aluminium oxide taut on said soda glass ring and then stretching said aluminium oxide film so that it acquires a permanent slackness.

l0. A method of making dynodes according to claim 9 wherein the step of stretching said aluminium oxide film so that it acquires a permanent slackness comprises heating said aluminium oxide film and said soda glass ring to a temperature such that said aluminium oxide film is stretched past the yield point thereof.

11. A method of making dynodes according to claim 10 wherein said aluminium oxide film and said soda glass ring are heated to a temperature in the range of about 300 to 500 C.

12. A method of making an electron discharge tube having incorporated therein at least one film type dynode said method comprising the steps of attaching across a supporting frame a slack, thin, supporting film, forming on one surface thereof a coating of secondary emission material, said coating of secondary emission material being of smaller dimensions than the area enclosed within the frame and being so situated Within the frame that a margin is left between the outer edge of the secondary emission material and the inner edge of the frame, mounting said structure within an electron discharge tube, and baking the tube to outgas it and at the same time to take up at least some of the slack in said supporting film by shrinkage of the coating of secondary emission material, and sealing said tube.

13. A method of making an electron tube having at least one lm type dynode mounted therein, said method comprising attaching across a soda glass circular ring supporting frame a thin film of aluminium oxide, coating said aluminium oxide with aluminium, providing slack in said composite film by heating the same beyond its yield point and then permitting the same to cool, thereafter forming on one surface of said composite film a coating of potassium chloride, and spacing said coating from said frame to provide annular margin between its outer edge and the inner edge of said frame, mounting said structure within an electron tube, and baking said tube while evacuating the same, whereby the tube is outgassed and the potassium chloride coating layer is shrunk so that at least some of the slack in said composite film is taken up, and sealing said tube.

References Cited in the le of this patent UNITED STATES PATENTS 1,699,597 Lederer lan. 22, 1929 2,624,024 Jansen Dec. 30, 1952 2,677,873 Buck May 11, 1954 2,708,788 Cassman May 24, 1955 2,721,372 Levi Oct. 25, 1955 2,746,129 Christensen May 22, 1956 2,871,086 Korner et al. Jan. 27, 1959 2,881,343 Polkosky et al. Apr. 7, 1959 2,942,133 McGee June 21, 1960 

1. A METHOD OF MAKING DYNODES FOR INCORPORATION IN ELECTRON DISCHARGE TUBES, SAID DYNODES CONSISTING OF AN OUTER FRAME AND A COMPOSITE FILM SUPPORTED BY SAID OUTER FRAME, SAID COMPOSITE FILM INCLUDING AN ELECTRON-PERMEABLE SUPPORTING LAYER AND A LAYER OF SECDONDARY EMISSION MATERIAL, SAID METHOD COMPRISING THE STEPS OF PRODUCING ON THE SUPPORTING FRAME A SLACK, THIN, SUPPORTING FILM, FORMING ON ONE SURFACE THEREOF A COATING OF SECONDARY EMISSION MATERIAL, SAID COATING OF SECONDARY EMISSION MATERIAL BEING OF SMALLER DIMENSIONS THAN THE AREA ENCLOSED WITHIN THE FRAME AND BEING SO SITUATED WITHIN THE FRAME THAT A MARGIN IS LEFT BETWEEN THE OUTER EDGE OF THE SECONDARY EMISSION MATERIAL AND THE INNER EDGE OF THE FRAME SO THAT, ON SUBSEQUENT BAKING, AT LEAST SOME OF THE SLACK IN THE SUPPORTING FILM IS TAKEN UP BY SHRINKAGE OF SAID ELECTRON EMISSION MATERIAL. 